With the recent development of the digital technology and improvement of data compression technologies, optical disks such as DVDs (digital versatile disks) have been attracting attention as a record medium for recording information such as music, movies, photographs, computer software, etc. (hereinafter also referred to as “contents”). As the price of optical disks goes down, optical disc apparatuses that use the optical disks as information recording medium have spread in the market.
The size of information of contents tends to increase year by year, which leads to an expectation for a further increase in the recording capacity of optical disks. As a means to increase the recording capacity of an optical disk, the record layer may be made into a multi-layered structure. Extensive developments are underway with respect to the optical disks having a plurality of recording layers (hereinafter also referred to as multi-layered disks) and also with respect to the optical disc apparatuses for accessing such multi-layered disks.
In the multi-layered disks, a wide gap between the record layers may cause deterioration of a signal reproduced from a selected record layer due to the effect of spherical aberration. There is thus a trend toward the narrowing of gaps between the record layers. As the gaps between the record layers narrow, however, inter-layer crosstalk occurs, so that a light beam returning from the multi-layered disk ends up including not only reflected light (hereinafter referred to as signal light) coming from the selected recording layer but also a significant amount of reflected light (hereinafter referred to as aberrant light) coming from other recording layers different from the selected recording layer. The S/N ratio of the reproduced signal may thus deteriorate.
In consideration of this, devices have been developed for reducing inter-layer crosstalk at the time of reproduction from a multi-layered disk (e.g., Patent Document 1 through Patent Document 3).
The apparatuses disclosed in Patent Document 1 through Patent Document 3, however, allow signal light and aberrant light to interfere with each other prior to reaching the photo detecting surface of the photo detecting device. This causes fluctuation in the amount of received light with respect to individual photo detecting devices, which may result in a drop in the S/N ratio of the reproduced signal.
[Patent Document 1] Japanese Patent Application Publication No. 2001-273640
[Patent Document 2] Japanese Patent Application Publication No. 10-11786
[Patent Document 3] International Patent Application Publication No. WO96/20473
Accordingly, there is a need for an optical pickup apparatus that can separate reflected light components from each other with sufficient precision when the reflected light components come from two recording layers of an optical disk.
There is another need for an optical disc apparatus which can reproduce information with sufficient precision when the information is obtained from an optical disk having two recording layers.